An interesting post - i too have an A100, and actually replaced it with a JDS Labs Atom. I my HD800s have hiss on the A100, and i always felt a little underwhelmed with the cans on that am. Picked up the Atom the other day for $50, and i love it in every way more than the A100. The signal is dead quiet, and i feel like the overall sound is just cleaner and more dynamic. Before getting the new amp, i was doubting my Hd800, but now...
As for the other question - I have three cans, HD650, LCD-2, and HD800 (my most recent grab from a pawn shop of all places). My 650's, which i have owned for years go with me to work, where i use them daily. And while i love them, at the end of the day, i still have the itch to fire up the HD800, because i feel they are that much better (more so with my new amp). I was never disappointed per se with my 650; i just couldn't say no to the 800 at the price offered.